The use of a separator between an anode and cathode in batteries, fuel cells, and electrochemical cells is known. In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemistry within the cell. Particularly, for example, in diaphragm chlorine generating cells, such a separator functions to restrain back migration of OH.sup.- radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH.sup.- back migration has been found to significantly decrease current inefficiencies associated with a reaction of the OH.sup.- radical at the anode releasing oxygen.
More recently separators based upon an ion exchange copolymer have found increasing application in batteries, fuel cells, and electrochemical cells. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons or perfluorocarbon copolymers and marketed by E. I. duPont under the name NAFION.RTM..
Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers made from perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.
Many chlorine cells use a sodium chloride brine feedstock. One drawback to the use in such cells of perfluorocarbon separators having pendant sulfonyl fluoride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic formed in these cells, including OH.sup.- radicals, from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce oxygen. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carbonyl based cation exchange groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0. 5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.
Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many batteries, fuel cells and electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating these electrodes remotely positioned on opposite sides of the cell separator.
Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. Typical sandwich SPE cells are described in U.S. Pat. Nos. 4,114,301; 4,057,479; 4,056,452 and 4,039,409.
At least one difficulty has surfaced in the preparation of SPE sandwiches employing porous reticulate electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally porous rectilinear electrode into one surface of a perfluorocarbon copolymeric membrane. In some instances, a second similar electrode is simultaneously or subsequently pressed into the observe membrane surface. To avoid heat damage to the copolymeric membrane, considerable pressure, often as high as 6000 psi is required to embed the electrode firmly in the membrane. For reasons related to reticulate electrode structural configuration, such pressure is generally required to be applied simultaneously over the entire electrode area, requiring a press of considerable proportions when preparing a commercial scale SPE electrode. As yet, the solution coating of such electrodes with perfluorocarbon copolymer has not been feasible principally due to difficulties in developing suitable solvation techniques for perfluorocarbon copolymer.
Microporous filters find considerable utility in the removal of minute particulates from liquid streams. Particularly, in the processing of chemicals generally in a solvated state such as in an aqueous medium for use, as an example, in the fabrication of microelectronics components such as computer memory chips, microporous filters have enjoyed considerable utility in preparation of such chemicals for use in fabricating the microelectronics components. Microscopic particulate contaminants, present upon a variety of microelectronic devices can cause a quality control related rejection of the device after manufacture, or dysfunctional field performance where the microelectronic device including such particulate contaminants becomes included as a component in a finished product. Microfiltration techniques applied to fluid streams utilized for fabricating such microelectronic-devices can improve the reliability of manufacturing processes producing such microelectronic-devices by removing substantially all microscopic particles present in the fluid stream.
Microporous filters can be fabricated from a variety of materials. Regardless of the material of fabrication, however, any resulting microporous filter should be openly porous, that is liquid should be capable of passing through the microporous filter from one surface to an obverse surface. Liquid passes through the filter via a plurality of interconnecting pores permeating the filter.
Typically, these pores are of a relatively uniform size, having for microporous filtration purposes, an average pore diameter of about 10 microns or less, and preferably having an average pore diameter of less than about 5 microns. Desirably, pores substantially deviating from the average, and particularly substantially larger pores are quite scarce or nonexistent to ameliorate the opportunity for undesirable particulates to pass through the filter. Conversely, where a large proportion of pores are substantially smaller than the average pore size, an unacceptably elevated resistance to flow through the microporous filter can result.
Microporous filters can be fabricated in a variety of well known manners. Typically, a large plurality of fibers, fibrils, or particles are formed into a desired filter shape and then provided with porosity. Often the particles, fibrils, or fibers are compounded with a pore precursor and tightly compressed to form a desired filter structure or substrate shape. The pore precursor is then removed, often by leaching, vaporization or the like.
While a microporous filter may be fabricated from a relatively wide selection of materials, where the microporous filter is to perform in an aggressive environment, the microporous filter must be capable of withstanding the aggressive environment. So, for example, where chemically an aggressive fluid including undesirable particulates is being passed through a microporous filter, the material of construction of the microporous filter should be capable of substantially resisting aggressive attack by the fluid being filtered. Typically, the filter in such applications is fabricated from a thermoplastic resin resistant to aggressive attack from the fluid being filtered. Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (KYNAR.RTM., a product of Pennwalt), polypropylene, polyethylene, and polysulfone from time to time may find utility in the fabrication of filters for microporous filtration.
When filtering fluids of a substantially aqueous nature, one drawback to the use of many thermoplastics in fabricating microporous filters is a tendency for many of the thermoplastics to be hydrophobic. Where aqueous materials must pass through micron sized pores in a filter, surface tension effects attributable to the hydrophobic nature of the thermoplastic filter can contribute to a substantial initial resistance to fluid flow through the filter quite apart from any resistance to fluid flow attributable to pluggage of the filter by particles being removed from the fluid or any pressure drop associated with mere hydraulic resistance to fluid flow through a small oriface such as a pore. Similarly, in operations of diaphragm type chlorine cells wherein a porous diaphragm separates anode and cathode compactments within the cell, the use of such hydrophobic polymers as PTFE has been proposed to provide such diaphragms with improved resistance to aggressive attack by contents of the cell. Hydrophobic properties of such a diaphragm can interfere with ordinary movement of contents of the cell from anode to cathode compactments.
Various attempts to ameliorate the hydrophobic nature of microporous filters made using hydrophobic material such as PTFE, KYNAR, and polypropylene thermoplastics have met with somewhat limited success. In one proposal, a surfactant has been applied to a microporous filter substrate in an effort to make the microporous filter more wettable through a reduction in surface tension effects that promote an elevated resistance to initial fluid passage through the microporous filter. Perhaps at least inpart due to the very resistant nature of such thermoplastics to aggressive chemical attack, or perhaps due to some aggressive attack upon the surfactant by the fluid being filtered, typically these surfactants were not well retained upon the microporous filter substrate, and would contaminate fluid flowing through the filter, contamination being inacceptable for fluids used in certain pharmaceutical and electronics applications.
In another proposal, chemical functional groups tending to impart hydrophilic properties to thermoplastics were engrafted to the thermoplastic materials by techniques such as radiation grafting and the like. Subsequent compounding and processing of the engrafted thermoplastic particles under heat to form a microporous filter substrate tended to degrade any hydrophilic properties imparted to the thermoplastic by virtue of the engrafted functional groups.
It is known that hydrophilic properties can be imparted to otherwise hydrophobic substrates by the application of a coating including chemical functional groups providing or tending to provide hydrophilic properties, the coating being applied to surfaces of the substrate. Such applied coatings of necessity should be relatively stable upon the hydrophobic substrate, that is tending to remain upon the substrate surfaces, and should be relatively immune to aggressive attack by any compounds contacting the coated substrate, attack being by corrosion, dissolution, reaction or any other process. Any functional groups providing hydrophilic properties associated with the coatings should likewise be relatively immune to aggressive attack from the compound contacting the coated substrate.
At least one material, perfluorocarbon copolymer has been suggested for use in coating porous substrates (U.S. Pat. No. 3,692,569). These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinylether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers usually containing an SO.sub.2 F, that is a sulfonyl fluoride group, or a group including or derived from COF, that is carbonyl fluoride. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F or CF.sub.2 .dbd.CFR.sub.1 COF. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising generally 1 to 8 carbon atoms but occasionally as many as 25 carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F or COF, particularly where the functional group exists as the --(--SO.sub.2 NH).sub.m Q form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups. Typical methyl carboxylate containing monomers are set forth in U.S. Pat. No. 4,349,422.
It has been suggested that such copolymeric perfluorocarbons can be applied to porous substrates by encapsulating the porous substrate in a film of the perfluorocarbon copolymer and applying heat until the perfluorocarbon copolymer thermoplastically softens and flows into the porous substrates to substantially coat all surfaces of the porous substrate. Functionality of the perfluorocarbon copolymer providing desirable hydrophilic properties to the coating are relatively readily destroyed by exposure to temperatures in excess of about 300.degree. C. The perfluorocarbon copolymer materials, however, are generally substantially viscous even at 300.degree. C. and do not satisfactorily penetrate a microporous structure to provide an essentially continuous coating upon particularly surfaces internal to microporous infrastructure of the microporous structure while not closing off open microporosity due to an excessively thick coating upon walls of the pores. Particularly for copolymeric perfluorocarbon wherein the functional group includes a Li.sup.+ or another cation salt of hydrolyzed SO.sub.2 F, elevated temperatures required for producing desired thermoplastic flow characteristics in this copolymeric perfluorocarbon may be in excess of a temperature at which significant thermal decomposition of the perfluorocarbon copolymer and/or its functionality commences.
In still another suggestion, perfluorocarbon copolymer having pendant SO.sub.2 F functional groups has been dissolved in a solvent to yield a solution which has been applied to a porous substrate. Any copolymeric perfluorocarbon so applied to microporous substrate that must remain openly microporous following coating application such as intended for performing a filtration function in an aqueous environment, should be of a sufficiently elevated equivalent weight to be retained upon the substrate and not gradually dissolved away by the aqueous fluid. Generally, the perfluorocarbon copolymer applied to, for example, microporous filters used in aqueous filtration should be of an equivalent weight of at least 900 to avoid dissipation by the aqueous fluid with an upper equivalent weight limitation being established by the minimal requisite pendant functionality necessary to impart desirably hydrophilic properties to the microporous substrate, generally an equivalent weight of about 1500. Further, where coating a microporous substrate, the solution of the perfluorocarbon copolymer must of necessity be of a sufficiently low viscosity at a suitably elevated content of the perfluorocarbon to assure its penetration of any microporous infrastructure of the microporous substrate to provide a generally uniform coating upon all the microporous substrate surfaces imparting a sufficient quantity of the perfluorocarbon copolymer to all the surfaces of the microporous substrate.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of at least partially solvated perfluorocarbon dispersions and for solution coating electrodes with the copolymer perfluorocarbon where the copolymeric perfluorocarbon is of a relatively elevated equivalent weight and is possessed of a functional group ionic form desirable for use in, for example, chlorine cell membranes and microporous filter coatings, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for dispersing and/or solvating these higher equivalent weight perfluorocarbons.
At more elevated equivalent weights, perfluorocarbon copolymer contains PTFE (polytetrafluoroethylene) like crystallinity. As is well known in polymer chemistry, once crystalline polymer materials commences appearing in a copolymer, dissolution becomes substantially more difficult. While temperature elevation is a frequently useful tool in such situations, with perfluorocarbon copolymers having pendant cation exchange functional groups, the usefulness of temperature elevation may be substantially limited. Known solvents for low equivalent weight copolymeric perfluorocarbons generally are possessed of a relatively low boiling point limiting the extent to which temperature elevation can be employed. In addition perfluorocarbon copolymer demonstrates a temperature degradation characteristic beginning to be significant at between about 250.degree. C. and 300.degree. C. or less.
For perfluorocarbon copolymers having pendant sulfonyl fluoride functionality, crystallized PTFE-like material begins to appear in the copolymer at between about an equivalent weight of 910 and 1050. Further, as described by Yeo in "Solubility Parameter of Perfluorosulfonated Polymer", perfluorocarbon solubility apparently is a function of the equivalent weight, becoming of substantial consideration above an equivalent weight of between about 910 and 1050 for sulfonyl fluoride functionality. Therefore solvents functioning upon lower equivalent weight material would appear not likely to function adequately at more elevated equivalent weight. Other articles such as: Seko et al "Perfluorocarboxylic Acid Membrane and Membrane Chlor-alkali Process Developed by Asahi Chemical Industry", Gierke et al "Morphology of Perfluorosulfonated Membrane Products", and Hashimoto et al "Structure of Sulfonated and Carboxylated Perfluorinated Ionomer Membranes", collected in Eisenberg et al "Perfluorinated Ionomer Membranes", Yomigama et al "Paper at No. 5 Caustic Soda Technical Forum, Kyoto Japan 11/81" and Starkweather "Crystallinity in Perfluorosulfonic Acid Ionomers and Related Polymers" further describe this phenomenon.
A method for reliable, facile application of a perfluorocarbon copolymeric coating to porous structures such as reticulated cell electrodes, cell diaphragms, and particularly to microporous substrates suitable for use in microporous filtration could find considerable acceptance in processes for the manufacture of such devices.